THURSDAY, July 15 (HealthDay News) -- A new study finds that 8 percent of gay male couples surveyed in the San Francisco area have differing views about whether their relationships are monogamous -- potentially putting some men at an unknowing risk of being infected with HIV.

The survey of 566 couples found that 45 percent said they were monogamous and slightly more said they were in open relationships. Other studies have come up with similar numbers, noted study lead author Colleen C. Hoff, a sexuality researcher.

However, the finding that a sizeable minority of partnered gay men are confused as to whether their bond is monogamous or not is a new finding, she said. "If one is having sex with someone outside the relationship, and the other doesn't know it, it could be an opportunity for risk [of HIV infection] for both partners," said Hoff, director of San Francisco State University's Center for Research on Gender and Sexuality.

Hoff and colleagues found the participants for the 2005-2007 survey through bars, coffeehouses, restaurants and advertisements in the San Francisco area.

Researchers asked the couples to separately answer questions about their sex lives. The researchers didn't tell the men about the answers given by their partners.

All but a tiny number of the couples reported having an agreement regarding sex: 45 percent agreed that they were in monogamous relationships, and 47 percent agreed that their relationships were open -- they could sleep with other people.

But in 8 percent of the couples, one partner thought the relationship was open and the other thought it was monogamous.

It wasn't necessarily true that partners were being dishonest, she said. "We just know that they have a different understanding of what the agreement is. It could be they weren't very clear from the beginning. It could be that their needs and desires have changed but they didn't talk about it, or they may have drifted away from the agreement and assumed the partner had, too."

The researchers also found the couples who agreed on the ground rules didn't necessarily create agreements -- and follow them -- in order to prevent HIV infection. "The primary motivating force is that these couples want to have a good relationship," she said.

Garrett Prestage, an HIV researcher in Australia, said the study findings should be interpreted cautiously because researchers may make assumptions based on their understanding of relationships among heterosexual couples.

Even so, Prestage said the research may lead to better understanding of sexual relationships between gay male couples and how they may affect their risk of HIV infection.

The findings show that it's wrong to assume that gay male couples make decisions about their relationships based on their concerns about HIV, said Prestage, an associate professor at the National Center in HIV Epidemiology & Clinical Research at the University of New South Wales.

"What matters in any relationship is the trust and interdependence of those involved," he said. "Rational considerations about infections, and most other things, usually take a backseat to these."

He added: "Unfortunately, most HIV prevention seems to be predicated on a message that implies they (gay men) should not trust their partners and should always act out of self-interest. That runs contrary to most healthy relationships."

The study, funded by the U.S. National Institute of Mental Health, is published in the July issue of the journal AIDS Care.